The present invention relates to clamping assemblies for injection molding apparatus, and, more particularly, to structures which ensure that the two portions of the injection molding mold remain together during the injection of molten plastic.
A typical injection molding machine features a fixed platen and a complementary movable platen. The two platens are designed to come together so as to form between them a multi-part, typically a two-part, mold. The two platens are pressed together through the use of a suitable driven ram, such as a hydraulic piston or toggle mechanism, and the like. With the two parts of the mold held firmly together, a molten plastic is injected under great pressure into the mold, thereby filling it. Once the plastic has cooled and hardened, the two parts of the mold are separated and the finished item is removed prior to repetition of the cycle.
Because of the high pressures under which the molten plastic is injected it is imperative that the two parts of the mold be very firmly and accurately held together. Thus, in order to prevent the escape of molten plastic and the attendant deterioration in the quality of the injection molded product, and in order to safeguard the mold itself, it is essential that the two parts remain accurately aligned with each other both while the two parts are pushed together and under application of high pressure.
For many years, and to a large extent even today, the accurate alignment of the fixed and movable platens was assured through the use of four tiebars. In other words, the square or rectangular movable platen features openings, typically near each of its four corners. Each opening slidably accommodates a tiebar which is firmly attached to the fixed platen. The movable platen is thus slidably supported by the tiebars which assure the proper alignment of the two platens even when the two platens are forced together under great pressure. In some cases, additional support is provided for the platens.
While the tiebars are effective in preserving the alignment of the platens, they do so at great cost. Thus, the presence of the tie bars in the region of the mold significantly reduces the portion of the mold available for molding and, perhaps more importantly, considerably complicates the manual or automatic removal of the product as well as the changing of the molds.
Several attempts have been made to provide an injection molding clamping assembly which dispenses with the troublesome tiebars. One of these is disclosed in U.S. Pat. No. 5,354,196 to Ziv-Av et al.
U.S. Pat. No. 5,354,196 discloses a clamping assembly which includes a base to which the fixed platen is connected and a clamping structure which nests within the base and which supports the movable platen. The base and clamping structure are arranged so that when pressure is applied to force the two platens together, the clamping structure absorbs the force, keeping the base from bending and ensuring a tight fit between the two platens.
This system maintains exact alignment between the platens by providing a double structure in which the base is isolated from the deforming effects of the clamping forces. Transmission of the clamping forces to the platens is achieved through two pivotal joints.
While this configuration represents a great improvement over the tiebar-based approach, the structure is somewhat complex and presents a number of practical limitations. Most notably, it has been found difficult to provide pivotal joints to reliably transmit the huge clamping forces required while preserving rotational freedom between the base and clamping structure.
In an alternative embodiment, U.S. Pat. No. 5,354,196 discloses a clamping system in which a single elongated base frame both absorbs the clamping forces and serves as a base for sliding bearings to preserve accurate alignment of the platens. Precision is achieved by generating a limited region of deformation remote from the sliding bearings.
The latter configuration avoids the shortcomings of the previously mentioned pivotal joints. However, the greatly extended base frame length required to achieve the isolation of the bearings from the region of deformation renders the system somewhat impractical. Furthermore, the positioning of the bearings remotely from the actual clamping position leads to a serious reduction in the accuracy of the clamping.
There is therefore a need for, and it would be highly advantageous to have, a reliable and tiebar-less clamping assembly for use in an injection molding apparatus which effectively maintains precise alignment of the platens without requiring pivotal connections or excessive size.